New Peerless faucet, water comes out shower head and faucet

I have just installed a peerless faucet and shower head in my bathroom. It is the kind that has the single handle control for both the water pressure and temperature. I got it all hooked up yesterday and went to test it and when I turn the water on, it comes out the faucet as well as dribbles out the shower head as well. If i pull the plunger thing up on the faucet, the shower operates fine. I am using 1/2 PEX to do all of my connections. Does anyone have any idea why it would be doing this? Thank you for your help!

. I am using 1/2 PEX to do all of my connections. Does anyone have any idea why it would be doing this? Thank you for your help!

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Don't use pex to connect the spout and shower pipes. The fittings will be small enough to restrict the flow from the spout and allowing it to go up the shower pipe.
Always use a full port rigid tubing such as copper for the faucet assembly. You can run your pex to the hot and cold sides only.

The simple pull-up divertors rely on the fact that the flow out the spout is normally wide open and unrestricted. Pex is smaller in diameter, and the flow rate can be larger than can freely flow through the pex and out the tub spout. If that happens, then water backs up and can dribble out the showerhead. It's sort of like putting your finger over the end of a hose, but it backs up and goes out the showerhead instead - where there's less restriction. Normally, the height of the showerhead riser is enough so it's easier to go out the tub spout, but not if you use pex. I've only installed a few, but it is very clear in the instructions of those I've installed...do NOT use pex out of the valve, or at least for the tub spout.

PEX will not ALWAYS cause the problem but it will happen often enough that you should not temp fate and use it for the tub spout. Besides the PEX pipe being smaller, the real problem is the EXTREME restriction caused by the fittings being inserted INTO the tubing.